Group IV and Group IV-VI Semiconductor Heterojunction Devices

ABSTRACT

A semiconductor PV detector comprises a Ge layer and a Pb-chalcogenide layer coupled to the Ge layer. The Ge layer comprises a first conduction band with a first conduction potential and a first valence band with a first valence potential. The Pb-chalcogenide layer comprises a second conduction band with a second conduction potential that is lower than the first conduction potential and a second valence band with a second valence potential that is lower than the first valence potential. The Ge layer and the Pb-chalcogenide layer form a heterojunction configured to allow electrons to flow from the Ge layer to the Pb-chalcogenide layer and allow holes to flow from the Pb-chalcogenide layer to the Ge layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This claims priority to U.S. Prov. Patent App. No. 62/888,164 filed onAug. 16, 2019, which is incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract NumberW911NF18104418 awarded by the Army Research Office of the Department ofDefense. The government has certain rights in the invention.

BACKGROUND

The commercial and dual-use (excluding military) world-wide market forinfrared imaging equipment has been estimated at well over $2 billionper year. Military applications for infrared imaging are estimated atabout $6 billion per year. There is a huge demand for a low-cost,uncooled camera with fast response time, especially for two-colorSWIR-MWIR and multi-color SWIR-MWIR-LWIR cameras.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a bandgap alignment of a heterojunctionof a group IV-VI Pb-chalcogenide semiconductor PV detector.

FIG. 2A is a schematic drawing of a bandgap alignment of PbSe/Ge.

FIG. 2B is a schematic drawing of a carrier flow of a Pb₁₋₁Sn_(x)Se/Geheterojunction, where x is the Sn composition that ranges from 0 toabout 0.35.

FIG. 3 is a schematic drawing of a group IV-VI Pb-chalcogenidesemiconductor on a Ge heterojunction detector.

FIG. 4 is a schematic drawing of a group IV-VI Pb-chalcogenidesemiconductor and a Ge heterojunction two-color detector.

FIG. 5 is a schematic drawing of a group IV-VI Pb-chalcogenidesemiconductor and a Ge_(x)Sn_(1-x) heterojunction two-color detector ona Ge substrate.

FIG. 6 is a schematic drawing of a group IV-VI Pb-chalcogenidesemiconductor and Ge heterojunction two-color detector on an Sisubstrate.

FIG. 7 is a schematic drawing of a group IV-VI Pb-chalcogenidesemiconductor and Ge_(x)Sn_(1-x) heterojunction two-color detectors onan Si substrate.

FIG. 8 is a schematic drawing of a group IV-VI Pb-chalcogenidesemiconductor and a Ge_(x)Sn_(y)Si_(1-x-y) heterojunction detector on anSi substrate.

FIGS. 9A and 9B show the RHEED pattern of a 1 μm PbSe film grown on a Gesubstrate.

FIG. 10A shows a surface SEM image of PbSe grown on Ge.

FIG. 10B shows cross-sectional SEM images of PbSe grown on Ge.

FIG. 11 is a room-temperature J-V curve of n-PbSe/p-Ge.

FIG. 12 shows PbSe/Ge current density compared to MCT rule 07.

FIG. 13 is a flowchart illustrating a method of fabricating asemiconductor PV detector.

DETAILED DESCRIPTION

Infrared imaging systems currently in use are costly, large, and heavy,and they consume large amounts of power. For example, with theproliferation of small-sized UAVs, it is desirable to develop a sensortechnology with costs that are in line with the overall cost of thevehicle. Currently, the only commercially-available, low-cost IR imagersare microbolometer arrays operating around a 9 μm wavelength. Inaddition, the third generation of IR detection modules is expected toprovide new functionalities such as multi-color capability. Simultaneousimaging in multiple wavelength bands across the spectrum enables one to“see” details far beyond the capabilities of the human eye. Manyelectro-optic/infrared MSI systems for ISR applications combine at leastone reflective-band/EO sensor with one thermal band/IR sensor. Fornight-vision imaging, it is desirable to have a multi-spectral system toprovide a combination of SWIR (1.0-1.7 μm), MWIR (3.5-5.0 μm), and LWIR(8-12 μm) bands into a single system. Visible light, NIR, and SWIR areusually considered EO bands and form images using reflected light from atarget. MWIR and LWIR are thermal IR bands, and they directly imageblackbody radiation from a target.

Currently, the leading technologies for two-color MWIR/LWIR thermalimaging systems include MCT, QWIP, and antimonide SL detection modules.Monolithically-integrated SWIR EO sensors with MWIR/LWIR sensors are notavailable. Using separate FPAs for different bands in the MSI systemoften requires complicated optical systems that increase size and cost.Therefore, having monolithically-integrated SWIR, MWIR, or LWIR FPAs onthe same chip is much desired. It is, however, very difficult tointegrate the SWIR EO band into any of the above-mentioned leadingMWIR/LWIR thermal imaging systems and maintain high performance due to amismatch of dissimilar materials. Another challenge for the currentleading material systems is a lack of a low-cost, large substrate. Forexample, a CdZnTe substrate for MCT is brittle and expensive. An Si or aGe substrate is preferred because it is scalable, cheap, andenvironmentally friendly. In addition, cryogenic cooling is required forthese sensors to have high performance, and that increases the SWaP.

In recent years, high-peak detectivity of 4.2×10¹⁰ cm·Hz^(1/2)·W⁻¹ and2.8×10¹⁰ cm·Hz^(1/2)·W⁻¹ at room temperature with and without ananti-reflection layer were reported. TE-cooled 240×320 format arrays ofPbSe photoconductor FPA monolithically fabricated on Si ROIC were alsoreported. These reports generated renewed interest in using a PbSedetector for high-temperature imaging applications in the mid-IR region.

To further improve the performance and overcome the drawbacks of thePb-salt photoconductor such as high 1/f noise and poor pixel statisticsdue to poor surface morphology, a CdS/PbSe heterojunction detector on anSi substrate has been proposed. However, the conduction band of CdS maybe higher than that of PbSe, which could block photo-generated carriers.For group IV-VI MWIR/LWIR semiconductors with bandgap, or energy gap,energies in the range of 0.1-0.3 eV, it is very challenging to findproper materials to form heterojunction structures for detectors. Aheterojunction is an interface between two layers or regions ofdissimilar semiconductors.

Disclosed herein are embodiments for group IV and group IV-VIsemiconductor heterojunction devices. The devices include MWIR detectorsand cameras. One non-limiting application is for two-color SWIR-MWIR andmulti-color SWIR-MWIR-LWIR cameras, which operate at high temperaturesand with fast response times. More particularly, the present disclosurepertains to a new heterojunction structure for group IV-VIPb-chalcogenide semiconductor PV detector fabrication. Where usedherein, the term “group IV” refers to the elements of IUPAC group IV(e.g., C, Si, Ge, Sn, and Pb); “group VI” refers to the elements ofIUPAC group VI (e.g., S, Se, and Te); and “chalcogenide” refers to achemical compound, including a sulfide, selenide, telluride, orpolonide, that comprises a chalcogen anion and an electropositiveelement. For instance, the heterojunction structure is a PbSe/Geheterojunction structure for an SWIR/MWIR two-color imaging applicationor an SWIR/MWIR/LWIR three-color multispectral imaging system. Thesystem can operate at TE-cooled temperatures. Ge is a part of theheterojunction, which enables monolithic fabrication of a large-formatFPA on a Ge or GeSi substrate. While a Pb-chalcogenide semiconductor isdiscussed, other semiconductor materials that provide MWIR detection,grow on group IV-VI materials such as Ge, and provide suitable bandalignment with those group IV-VI materials may be used.

Before describing various embodiments of the present disclosure in moredetail by way of exemplary description, examples, and results, it is tobe understood as noted above that the present disclosure is not limitedin application to the details of methods and apparatus as set forth inthe following description. The present disclosure is capable of otherembodiments or of being practiced or carried out in various ways. Assuch, the language used herein is intended to be given the broadestpossible scope and meaning; and the embodiments are meant to beexemplary, not exhaustive. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting unless otherwiseindicated as so. Moreover, in the following detailed description,numerous specific details are set forth in order to provide a morethorough understanding of the disclosure. However, it will be apparentto a person having ordinary skill in the art that the embodiments of thepresent disclosure may be practiced without these specific details. Inother instances, features which are well known to persons of ordinaryskill in the art have not been described in detail to avoid unnecessarycomplication of the description.

Unless otherwise defined herein, scientific and technical terms used inconnection with the present disclosure shall have the meanings that arecommonly understood by those having ordinary skill in the art. Further,unless otherwise required by context, singular terms shall includepluralities and plural terms shall include the singular.

All patents, published patent applications, and non-patent publicationsmentioned in the specification are indicative of the level of skill ofthose skilled in the art to which the present disclosure pertains. Allpatents, published patent applications, and non-patent publicationsreferenced in any portion of this application are herein expresslyincorporated by reference in their entirety to the same extent as ifeach individual patent or publication was specifically and individuallyindicated to be incorporated by reference.

As utilized in accordance with the methods and apparatus of the presentdisclosure, the following terms, unless otherwise indicated, shall beunderstood to have the following meanings:

The use of the word “a” or “an” when used in conjunction with the term“comprising” in the claims and/or the specification may mean “one,” butit is also consistent with the meaning of “one or more,” “at least one,”and “one or more than one.” The use of the term “or” in the claims isused to mean “and/or” unless explicitly indicated to refer toalternatives only or when the alternatives are mutually exclusive,although the disclosure supports a definition that refers to onlyalternatives and “and/or.” The use of the term “at least one” will beunderstood to include one as well as any quantity more than one,including but not limited to, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 30,40, 50, 100, or any integer inclusive therein. The term “at least one”may extend up to 100 or 1000 or more, depending on the term to which itis attached; in addition, the quantities of 100/1000 are not to beconsidered limiting, as higher limits may also produce satisfactoryresults. In addition, the use of the term “at least one of X, Y and Z”will be understood to include X alone, Y alone, and Z alone, as well asany combination of X, Y and Z.

As used herein, all numerical values or ranges (e.g., in units of lengthsuch as micrometers or millimeters) include fractions of the values andintegers within such ranges and fractions of the integers within suchranges unless the context clearly indicates otherwise. Thus, toillustrate, reference to a numerical range, such as 1-10 includes 1, 2,3, 4, 5, 6, 7, 8, 9, 10, as well as 1.1, 1.2, 1.3, 1.4, 1.5, etc., andso forth. Reference to a range of 1-50 therefore includes 1, 2, 3, 4, 5,6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, etc., up to andincluding 50, as well as 1.1, 1.2, 1.3, 1.4, 1.5, etc., 2.1, 2.2, 2.3,2.4, 2.5, etc., and so forth. Reference to a series of ranges includesranges which combine the values of the boundaries of different rangeswithin the series. Thus, to illustrate reference to a series of ranges,for example, of 1-10, 10-20, 20-30, 30-40, 40-50, 50-60, 60-75, 75-100,100-150, 150-200, 200-250, 250-300, 300-400, 400-500, 500-750,750-1,000, includes ranges of 1-20, 10-50, 50-100, 100-500, and500-1,000, for example. For example, a reference to a range of 3 mm and20 mm in diameter, or a range of 50 μm to 300 μm in thickness, isintended to explicitly include all units of measurement in the range.

As used herein, the words “comprising” (and any form of comprising, suchas “comprise” and “comprises”), “having” (and any form of having, suchas “have” and “has”), “including” (and any form of including, such as“includes” and “include”) or “containing” (and any form of containing,such as “contains” and “contain”) are inclusive or open-ended and do notexclude additional, unrecited elements or method steps.

The term “or combinations thereof” as used herein refers to allpermutations and combinations of the listed items preceding the term.For example, “A, B, C, or combinations thereof” is intended to includeat least one of: A, B, C, AB, AC, BC, or ABC, and if order is importantin a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB.Continuing with this example, expressly included are combinations thatcontain repeats of one or more item or term, such as BB, AAA, AAB, BBC,AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan willunderstand that typically there is no limit on the number of items orterms in any combination, unless otherwise apparent from the context.

Throughout this application, the terms “about” and “approximately” areused to indicate that a value includes the inherent variation of error.Further, in this detailed description, each numerical value (e.g.,temperature or time) should be read once as modified by the term “about”(unless already expressly so modified), and then read again as not somodified unless otherwise indicated in context. As noted above, anyrange listed or described herein is intended to include, implicitly orexplicitly, any number within the range, particularly all integers,including the end points, and is to be considered as having been sostated. For example, “a range from 1 to 10” is to be read as indicatingeach possible number, particularly integers, along the continuum betweenabout 1 and about 10. Thus, even if specific data points within therange, or even no data points within the range, are explicitlyidentified or specifically referred to, it is to be understood that anydata points within the range are to be considered to have beenspecified, and that the inventors possessed knowledge of the entirerange and the points within the range. Unless otherwise stated, theterms “about” or “approximately”, where used herein when referring to ameasurable value such as an amount, length, thickness, a temporalduration, and the like, is meant to encompass, for example, variationsof ±20% or ±10%, or ±5%, or ±1%, or ±0.1% from the specified value, assuch variations are appropriate to perform the disclosed methods and asunderstood by persons having ordinary skill in the art.

As used herein, the term “substantially” means that the subsequentlydescribed parameter, event, or circumstance completely occurs or thatthe subsequently described parameter, event, or circumstance occurs to agreat extent or degree. For example, the term “substantially” means thatthe subsequently described parameter, event, or circumstance occurs atleast 90% of the time, or at least 91%, or at least 92%, or at least93%, or at least 94%, or at least 95%, or at least 96%, or at least 97%,or at least 98%, or at least 99%, of the time, or means that thedimension or measurement is within at least 90%, or at least 91%, or atleast 92%, or at least 93%, or at least 94%, or at least 95%, or atleast 96%, or at least 97%, or at least 98%, or at least 99%, of thereferenced dimension or measurement (e.g., length).

As used herein any reference to “one embodiment” or “an embodiment”means that a particular element, feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment.

It should be understood at the outset that, although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

The following abbreviations apply:

A: ampere(s)

BaF₂: barium fluoride

C: carbon

CB: conduction band

Cd: cadmium

CdS: cadmium sulfide

CdZnTe: cadmium zinc telluride

cm: centimeter(s)

E_(f): Fermi level

E_(g): energy gap

EO: electro-optic

ETL: electron transport layer

Eu: europium

eV: electron-volt(s)

e⁻: electron

FPA: focal plane array

Ge: germanium

GeSi: germanium silicide

HTL: hole transport layer

Hz: hertz

h⁺: hole

IR: infrared

ISR: intelligence, surveillance, and reconnaissance

IUPAC: International Union of Pure and Applied Chemistry

J-V: current density-voltage

K: Kelvin

LWIR: long-wave IR

MBE: molecular-beam epitaxy

MCT: mercury cadmium telluride

MSI: multispectral imaging

MWIR: mid-wave IR

nm: nanometer(s)

NWIR: near-wave IR

Pb: lead

PbSe: lead selenide

PIN: p-type, intrinsic, n-type

PV: photovoltaic

QWIP: quantum well IR photodetector

RHEED: reflection high-energy electron diffraction

ROIC: read-out integrated circuitry

S: sulfur

Se: selenium

SEM: scanning electron microscope

Si: silicon

SL: superlattice

Sn: tin

Sr: strontium

SRH: Shockley-Read-Hall

SWaP: size, weight, and power

SWIR: short-wave IR

Te: tellurium

TE: thermoelectric

UAV: unmanned aerial vehicle

V: volt(s)

VB: valence band

W: watt(s)

μm: micrometer(s)

2D: two-dimensional

3D: three-dimensional.

FIG. 1 shows a schematic drawing of a group IV-VI Pb-chalcogenidesemiconductor PV double-heterojunction detector structure 100. The leftlayer serves as an ETL, the middle layer is a group IV-VIPb-chalcogenide semiconductor that serves as an MWIR absorption layer,and the right layer serves as an HTL. All three layers could absorblight at different wavelengths and form a three-color detector. GroupIV-VI Pb-chalcogenide semiconductors could be p-type, n-type, orintrinsic. The ETL may be n doped to encourage electrons in thePb-chalcogenide semiconductors to transport into the ETL, and the HTLmay be p doped to encourage holes in the Pb-chalcogenide semiconductorsto transport into the HTL. In such cases, an n-p junction will be formedbetween ETL and Pb-chalcogenide semiconductors, and a p-n junction willbe formed between HTL and Pb-chalcogenide semiconductors. Also, in someembodiments, both the ETL and the HTL have wider bandgaps than that ofthe Pb-chalcogenide semiconductors. Either the ETL or the HTL could bereplaced by ohmic contact material. In such cases, the doubleheterojunction becomes a single-heterojunction structure and a potentialtwo-color detector structure.

For group IV-VI Pb-chalcogenide narrow-bandgap semiconductors in whichthe bandgap energy is typically in the range of 0.1 eV to 0.4 eV, it ischallenging to find a proper ETL or HTL to form such heterojunctions. Inthe present disclosure, group IV semiconductors, such as Ge, are used asthe HTL for group IV-VI Pb-chalcogenide semiconductors such as PbSe.

FIGS. 2A and 2B show a schematic drawing of an example group IV andgroup IV-VI Pb-chalcogenide semiconductor heterojunction. Specifically,in FIGS. 2A and 2B, the heterojunction is a Ge and Pb-chalcogenideheterojunction. The electron affinity of both Ge and PbSe is wellstudied, which ensures the required proper band alignment to fabricateheterojunction detectors. Specifically, as shown in a band alignment 200in FIG. 2A, the conduction band of the Ge layer has a higher potentialthan the conduction band of the PbSe layer, and the valence band of thePbSe layer has a lower potential than the valence band of the Ge layer.Put another way, the conduction band of the PbSe layer is lower than theconduction band of the Ge layer, and the valence band of the Ge layer ishigher than the valence band of the PbSe layer. As shown in a carrierflow 210 in FIG. 2B, this allows electrons to flow from the Ge layer tothe PbSe layer and holes to flow from the PbSe layer to the Ge layer. Inaddition, the bandgap in the Ge layer provides for SWIR detection, andthe bandgap of the PbSe layer provides for MWIR detection. Since largeGe substrates are commercially available, the most convenient approachis to grow a thin film Pb_(1-x)Sn_(x)Se on a Ge substrate. The roomtemperature cut-off wavelength for PbSe is 4.6 μm. For Sn compositionwith x from 0 to about 0.35, the bandgap changes from 0.27 eV to zero,which covers the MWIR/LWIR range. Therefore Pb_(1-x)Sn_(x)Se with adifferent Sn composition allows fabrication of an MWIR or LWIR detector.

FIGS. 3-8 show schematic drawings of non-limiting embodiments ofstructures of such heterojunction PV detectors. The group IV-VIPb-chalcogenide semiconductors could be, for example, PbXSe, PbXTe,PbXS, where X represents a chemical element with a composition able toform ternary compound semiconductors. For example, X could be Sn, Sr,Eu, Cd, or Ge. The group IV-VI Pb-chalcogenide semiconductors could alsohave four or more elements such as PbXSe_(x)Te_(1-x), PbXSe_(x)S_(1-x),PbXSe_(x)S_(x)Te_(1-x-y), where x and y are the composition. The groupIV-VI Pb-chalcogenide semiconductors could be in mono-crystalline,polycrystalline, or amorphous form. The group lead salt materials couldbe in bulk form, micro-crystalline form, or nano-structure form such asquantum dot, quantum wire, or quantum well. They could be in 2D form or3D form such as wires. The group IV semiconductor could be a singleelement such as Ge or multiple elements such as Ge on Si. It could be agroup IV semiconductor alloy such as GexSn_(y)Si_(1-x-y). The group IVsemiconductor could be also grown on any substrate such as glass, BaF₂,quartz, sapphire, or a conductive transparent oxide.

In a detector 300 in FIG. 3, the detection wavelength is mainlydetermined by the Pb-chalcogenides semiconductor. The Ge layer servesbasically as a substrate. The Ge layer could also include Ge-basedintegrated circuitry.

In a detector 400 in FIG. 4, the Pb-chalcogenide semiconductor is grownon top of a Ge PIN detector. When contacts x05 and x06 are used toextract current, the structure serves as a Ge SWIR detector. Whencontacts x06 and x07 are used to extract current, the structure servesas a Pb_(1-x)Sn_(x)Se MWIR or LWIR detector depending on x. Whencontacts x05 and x07 are used to extract current, the structure cansimultaneously detect in the SWIR and MWIR/LWIR wavelength range. Athree-color detector structure could also be made with the topPb_(1-x)Sn_(x)Se being a two-layer tandem structure, one in MWIR andanother in LWIR.

A detector 500 in FIG. 5 is a more general case of the structure shownin FIG. 4. In this structure, two layers of the Ge in FIG. 4 (x02 andx03) are replaced by Ge_(x)Sn_(1-x) alloys. The Ge_(x)Sn_(1-x) alloyshave different material properties such as bandgap energy and latticeconstant. For example, the detector cutoff wavelength will be tuned bythe Sn composition in Ge_(x)Sn_(1-x) alloys.

In a detector 600 in FIG. 6, the two-color detector described in FIG. 4is monolithically fabricated on the Si substrate. Layer x02 can be Ge, aGe_(x)Si_(1-x) alloy, or their graded layers. Ge on Si technology iswell developed, and such wafers are commercially available. This allowsmonolithic integration of the two-color detector shown in FIG. 4 onto SiROIC.

A detector 700 in FIG. 7 is a more general case of the structure shownin FIG. 6. In this structure, two layers of Ge in FIG. 6 (x03 and x04)are replaced by Ge_(x)Sn_(1-x) alloys that have different materialproperties such as band structure and lattice constant. In this case,the two-color detector described in FIG. 5 is monolithically fabricatedon the Si substrate. This allows monolithic integration of the two-colordetector shown in FIG. 5 onto Si ROIC.

A detector 800 in FIG. 8 is a more general case of the structures shownin FIGS. 3-7. In this structure, group IV semiconductors are representedby Ge_(x)Sn_(y)Si_(1-x-y) alloys that can apply to all of the detectorstructure shown in FIGS. 3-7.

Additional embodiments may, for instance, improve material quality andthus junction characteristics. First, the mechanism of strain relaxationin group IV-VI materials is by glide of dislocations. The Burgersvectors are of type a/2<110> on the primary glide planes of {100}. For(100) oriented layers, the Schmid factors for glide in the primary {100}planes are zero, and no glide can occur in the main {100} glide system.Therefore, PbSe grown on [100] orientated dissimilar substrate oftencracks due to the mismatch of lattice constant and thermal expansioncoefficient. It has been proven that growth on non-(100) surface such as(111) surface could provide much higher material quality. (100) Gesubstrate may be used because of the lower cost of (100) substrates.Second, post-growth treatment can be used to passivate defects in thePbSe and PbSe/Ge interface. For instance, iodine and oxygen may be usedon PbSe. The passivation may reduce SRH recombination in a manner tosimilar to that for PbSe photodetectors.

Experimental Results

The key challenge for the structure was to grow high-quality PbSe on aGe substrate. To demonstrate the concept, n-type PbSe was grown on ap-type Ge substrate in a water-cooled MBE system. A p-type Ge wafer wasoriented with a mis-cut of 6° towards the nearest plane. After thermaltreatment to remove a Ge oxide layer, PbSe was grown on the Gesubstrate.

FIG. 9A shows the RHEED pattern 900 of PbSe grown on Ge (100) at the[100] azimuth after growth. FIG. 9B shows the RHEED pattern 910 of PbSegrown on Ge (100) at the azimuth after growth. The RHEED pattern showsthat PbSe follows the substrate orientation and forms single crystallinefilms with layer-by-layer growth.

FIG. 10A shows a surface SEM image 1000 of PbSe grown on Ge (100). FIG.10B shows cross-sectional SEM images 1010 of PbSe grown on Ge. Thesurface is crack-free with a very smooth surface. There are patternedsurface ripples of about 10 nm in depth, which are most likely caused bythe mismatch of lattices and therma expansion coefficients. The sampleis etched into 120 μm×120 μm pixels.

FIG. 11 is a room-temperature J-V curve 1100 of n-PbSe/p-Ge. The J-Vcurve was measured by a probe station. The J-V curve demonstrates a p-njunction diode characteristic with a rectifying factor of over 600 atroom temperature.

FIG. 12 is a graph 1200 showing PbSe/Ge current density compared to MCTrule 07. With only limited development, the J-V curve of PbSe/Ge hasexceeded the limits of the MCT detector.

FIG. 13 is a flowchart illustrating a method 1300 of fabricating asemiconductor PV detector. At step 1310, a substrate is obtained. Forinstance, the substrate is an Si substrate. At step 1320, a Ge layer isgrown on the substrate. The Ge layer comprises a first conduction bandwith a first conduction potential and a first valence band with a firstvalence potential. Finally, at step 1330, a Pb-chalcogenide layer isgrown on top of the Ge layer to form a heterojunction. ThePb-chalcogenide layer comprises a second conduction band and a secondvalence band. The second conduction band has a second conductionpotential that is lower than the first conduction potential. The secondvalence band has a second valence potential that is lower than thesecond valence potential. The heterojunction is configured to allowelectrons to flow from the Ge layer to the Pb-chalcogenide layer andallow holes to flow from the Pb-chalcogenide layer to the Ge layer. Asubstrate or layer defined by a material, for instance Ge, is a layer inwhich that material is the primary material. For instance, the Ge layerprimarily comprises Ge, but may further comprise other materials such asSn or Si.

The embodiments have several advantages over other approaches. First, GeSWIR detectors are a mature technology and are commercially available,so Ge and Pb-chalcogenide semiconductors can be monolithicallyintegrated on a Ge/Si substrate and naturally form a two-color SWIR-MWIRPV detector. Second, large, cost-effective Ge substrates arecommercially available, which enables fabrication of low-cost,large-format detector FPAs, which lead to low-cost SWIR-MWIR two-colorcameras. Third, the group IV semiconductor (e.g., Ge) is not only usedas a substrate, but is additionally an important functionalsemiconductor as an HTL and an SWIR absorber in the two-color detector.Fourth, the PbSe PV detector overcomes drawbacks of otherphotoconductors, namely large 1/f noise and inhomogeneity.

What is claimed is:
 1. A semiconductor photovoltaic (PV) detectorcomprising: a germanium (Ge) layer comprising: a first conduction bandwith a first conduction potential, and a first valence band with a firstvalence potential; and a lead-chalcogenide (Pb-chalcogenide) layercoupled to the Ge layer and comprising: a second conduction band with asecond conduction potential that is lower than the first conductionpotential, and a second valence band with a second valence potentialthat is lower than the first valence potential, wherein the Ge layer andthe Pb-chalcogenide layer form a heterojunction configured to: allowelectrons to flow from the Ge layer to the Pb-chalcogenide layer, andallow holes to flow from the Pb-chalcogenide layer to the Ge layer. 2.The semiconductor PV detector of claim 1, wherein the Ge layer has abandgap of about 0.66 electron-volts (eV).
 3. The semiconductor PVdetector of claim 1, wherein the Pb-chalcogenide layer has a bandgap ofabout 0.27 electron-volts (eV).
 4. The semiconductor PV detector ofclaim 1, wherein the Pb-chalcogenide layer is configured to detect afirst color in a mid-wave infrared (MWIR) range.
 5. The semiconductor PVdetector of claim 4, wherein the Ge layer is configured to detect asecond color in a short-wave infrared (SWIR) range.
 6. The semiconductorPV detector of claim 1, further comprising a first ohmic contact coupledto the Pb-chalcogenide layer.
 7. The semiconductor PV detector of claim6, further comprising a second ohmic contact coupled to the Ge layer. 8.The semiconductor PV detector of claim 1, wherein the Ge layercomprises: a p-type layer coupled to the Pb-chalcogenide layer; anintrinsic layer coupled to the p-type layer; and an n-type layer coupledto the intrinsic layer.
 9. The semiconductor PV detector of claim 8,wherein the p-type layer comprises tin (Sn).
 10. The semiconductor PVdetector of claim 9, wherein the intrinsic layer comprises Sn.
 11. Thesemiconductor PV detector of claim 10, wherein the Pb-chalcogenide layercomprises selenium (Se), tellurium (Te), sulfur (S), Sn, strontium (Sr),europium (Eu), cadmium (Cd), or Ge.
 12. The semiconductor PV detector ofclaim 11, further comprising a silicon (Si) substrate coupled to then-type layer.
 13. The semiconductor PV detector of claim 12, wherein then-type layer comprises germanium silicide (GeSi).
 14. The semiconductorPV detector of claim 8, further comprising a silicon (Si) substratecoupled to the n-type layer.
 15. The semiconductor PV detector of claim14, wherein the n-type layer comprises germanium silicide (GeSi). 16.The semiconductor PV detector of claim 1, wherein the Ge layer comprisestin (Sn).
 17. The semiconductor PV detector of claim 16, wherein the Gelayer further comprises silicon (Si).
 18. The semiconductor PV detectorof claim 17, further comprising an Si substrate coupled to the Ge layer.19. The semiconductor PV detector of claim 1, wherein the semiconductorPV detector is a camera.
 20. A method of fabricating a semiconductor PVdetector and comprising: obtaining a substrate; growing a germanium (Ge)layer on the substrate, wherein the Ge layer comprises a firstconduction band with a first conduction potential and a first valenceband with a first valence potential; and growing a lead-chalcogenide(Pb-chalcogenide) layer on top of the Ge layer to form a heterojunction,wherein the Pb-chalcogenide layer comprises a second conduction band anda second valence band, wherein the second conduction band has a secondconduction potential that is lower than the first conduction potential,wherein the second valence band has a second valence potential that islower than the first valence potential, and wherein the heterojunctionis configured to allow electrons to flow from the Ge layer to thePb-chalcogenide layer and allow holes to flow from the Pb-chalcogenidelayer to the Ge layer.